The present invention relates generally to electrical propulsion systems for traction vehicles, and it relates more particularly to means for providing improved electrical braking of a system using electric power choppers to control the magnitude of current in self excited d-c traction motors.
Large electrically driven traction vehicles such as locomotives or transit cars are propelled by a plurality of traction motors mechanically coupled to the respective wheel sets of the vehicle. Such motors are usually of the direct current (d-c) type. A d-c traction motor comprises a stator, a rotor, armature windings on the rotor, and field windings (either connected in series with the armature or separately excited) on the stator. In order to control its tractive effort, there is associated with the motor suitable means for regulating the magnitude of direct current in the motor armature. Electric power apparatus commonly known as a chopper is an energy conserving means for regulating armature current.
A chopper is essentially a controlled switch connected in circuit with the motor armature to meter current from a source of relatively constant d-c electric power to the motor. The switch is cyclically operated between open and closed states, and by appropriately controlling the timing of the successive transitions between these alternate states the magnitude of armature current can be varied or maintained substantially constant as desired. Assuming the chopper is in series with the motor and the propulsion system is operating in its motoring mode, during closed periods of the chopper the motor armature windings will be connected to the d-c power source through a path of negligible resistance, whereby virtually the full magnitude of the source voltage is applied to the motor armature and the current tends to increase. During the open periods of the chopper, the motor is disconnected from the power source and armature current, circulating through a free wheeling path, decays from the magnitude previously attained. In this manner, pulses of voltage are periodically applied to the motor, and an average magnitude of motor current (and hence torque) is established. The rate of change of current is limited by the circuit inductance.
The ratio of the closed time (t.sub.ON) of the chopper to the sum of the closed and open times (t.sub.ON +t.sub.OFF) during each cycle of operation is the duty factor of the chopper. For a 0.5 duty factor, the repetitive closed and open periods of the chopper are equal to each other, and the width of each voltage pulse has the same duration as the space between successive pulses. In practice, so long as the chopper frequency is relatively high (such as, for example, 300 Hz) the circuit inductance (including the inductance provided by the armature windings of the traction motor itself) will smooth the undulating current in the motor armature sufficiently to prevent untoward torque pulsations, whereby the vehicle is propelled without any uncomfortable amount of jerking or lurching. By varying the duty factor of the chopper, the average chopper output voltage (as a percentage of the d-c source voltage) and consequently the average magnitude of current can be increased or decreased as desired. This is popularly known as time ratio control or pulse control.
A propulsion system using choppers can be adapted for electrical braking by reconnecting the power circuits so that each chopper is connected to the d-c power source in parallel rather than in series with its associated motor. In the braking mode of operation, a traction motor behaves as a generator, and the magnitude of its generated voltage (electromotive force) is proportional to speed and field excitation. The excitation of a series field machine is a function of the magnitude of armature current. With the chopper reconnected in parallel with the motor, during its closed periods the chopper provides a low resistance path for armature current which therefore tends to increase, whereas during its open periods the armature current path includes the power source and the free wheeling path, whereby current tends to decrease. The electric power output of the motor is either fed back to the source (regenerated), or dissipated in a dynamic braking resistor grid that can be connected in parallel with the chopper, or a combination of both. In either case, the average magnitude of armature current (and hence braking effort) can be controlled as desired by varying the duty factor of the chopper.
In the present state of the art, choppers for traction vehicle applications use high-power, solid-state controllable switching devices known as thyristors or silicon controlled rectifiers (SCRs). A thyristor is typically a three-electrode device having an anode, a cathode, and a control or gate terminal. When its anode and cathode are externally connected in series with an electric power load and a source of forward anode voltage (i.e., anode potential is positive with respect to cathode), a thyristor will ordinarily block appreciable load current until a firing signal is applied to the control terminal, whereupon it switches from its blocking or "off" state to a conducting or "on" state in which the ohmic value of the anode-to-cathode resistance is very low. Once triggered in this manner and latched in by conducting load current of at least a predetermined minimum magnitude prior to removal of the firing signal, the thyristor can be turned off only by reducing the current through the device to zero and then applying a reverse voltage across the anode and cathode for a time period sufficient to allow the thyristor to regain its forward blocking ability. Such a device forms the main load-current-carrying switching element of the chopper, and suitable means is provided for periodically turning it on and off.
In practical applications the main thyristor of the chopper is periodically turned off by means of a "commutation" circuit connected in parallel therewith. A typical commutation circuit is a "ringing" circuit, i.e., the circuit contains inductive and capacitive components that develop an oscillating or ringing current. A chopper commutation circuit may include, for example, a precharged capacitor, an inductor, a diode, and the inverse parallel combination of another diode and an auxiliary thyristor. In a voltage turn-off type of chopper, these components of the commutation circuit are so interconnected and arranged so as to divert load current from the main thyristor in response to turning on the auxiliary thyristor, and the main thyristor current is soon reduced to zero. The ringing action of the commutation circuit temporarily reverse biases the main thyristor which is consequently turned off, and during the reverse bias interval the current in the auxiliary thyristor oscillates to zero so that the latter component will also be turned off. For an ensuing brief interval, load current will continue to flow through the capacitor and a series diode in the commutation circuit of the chopper, thereby recharging the capacitor from the d-c source to complete the commutation process. Now the chopper is in an open or non-conducting state, and it cannot return to its closed or conducting state until the main thyristor is subsequently turned on by applying another firing signal.
The duty factor or percentage on time of the chopper is determined by the time delay between firing the auxiliary thyristor and subsequently firing the main thyristor during any full cycle of operation. The shorter this delay, the higher the duty factor, whereas the longer this delay, the lower the duty factor. Practical limits are imposed by the nature of the switching devices used in the chopper. For example, the maximum duty factor is approximately 0.91 for a chopper using a main thyristor rated 1100 amps (average) and 2000 volts (peak forward voltage) and operating at a constant frequency of approximately 300 Hz. A higher duty factor cannot be safely obtained at that chopping frequency because the aforementioned time delay must be at least 300 microseconds to make sure that the main thyristor is not refired prematurely, i.e., before the auxiliary thyristor has time to be completely turned off during the commutation process. For the same assumed parameters, the minimum duty factor would be approximately 0.09. This is because the minimum pulse width per cycle is determined by the recharging time of the capacitor in the oscillatory commutation circuit. Consequently, so long as it is being operated in a constant frequency variable pulse width mode, the chopper is effective to control motor current only in a limited range between its predetermined minimum and maximum duty factors.
It is generally desirable to be able to vary the chopper duty factor over substantially the full range between 0 and 1.0. In U.S. Pat. No. 3,944,856, a constant frequency oscillator ordinarily determines the free running frequency of the chopper, but at high motor speeds a pair of frequency dividers are combined with appropriate logic components to effect a two-step reduced frequency, maximum pulse width mode of operation, thereby extending the range of duty factor variations above the maximum attainable when the chopper is operated in its constant high frequency, variable pulse width mode. At the lower chopping frequencies the minimum delay time required after turning on the auxiliary thyristor before refiring the main thyristor is a smaller fraction of the whole period of each cycle. By thus increasing the duty factor, the percentage of the available d-c source voltage that the chopper can apply to the motor armature is desirably increased. In the referenced patent the chopper frequency is reduced in two discrete steps that are just equal, respectively, to one-third and one-half of the constant high frequency, and this technique is not optimum for controlling armature current during low speed electrical braking of a chopper type propulsion system on a large traction vehicle.
Smooth continuous variations of the duty factor up to 1.0 are desirable during the braking mode of operation to obtain high, constant braking effort when the vehicle is traveling at low speeds. The higher the duty factor, the lower the minimum speed at which the maximum magnitude of armature current can be sustained during braking. Once the vehicle decelerates below this minimum speed, braking effort will decrease or fade out. The lowest possible minimum brake fade out speed is generally desirable.
Before changing from motoring to braking modes of operation, it is good practice to reduce the chopper duty factor to zero so that there is no current in the armature of the motor at the time the propulsion system is reconnected for braking operation. If the vehicle were moving slowly when the motoring-to-braking transition is desired, it would be difficult to turn on the chopper after the transition. This is because at low speeds the voltage generated by the motor is low, especially in a series fluid motor with zero current. The low voltage may be insufficient to forward bias the main thyristor in the chopper. Even if the main thyristor were successfully triggered, a relatively long time is required for current to build up to an appreciable level in the armature current path, and there is a possibility that latching current will not be attained during the period of the firing signal that is normally applied. Raising the voltage of the motor by using the prior art technique of boosting its field at the beginning of a braking mode of operation is helpful but does not completely solve the problem. Lengthening the period of the normal firing signals is not a desirable solution because of the attendant energy loss and isolation problems. Shunting the free wheeling path with an inversely poled auxiliary thyristor that temporarily conducts current from the d-c source for augmenting current flowing through the main thyristor when initially triggered, as suggested in prior art U.S. Pat. No. 3,748,560, is not a practical solution.